Precipitate size and morphology govern the strength–ductility trade-off in an additively manufactured medium-entropy alloy

Additively manufactured CoCrNiAlTi medium-entropy alloys were fabricated and subjected to aging treatments to regulate precipitate size and morphology. Neutron diffraction and small-angle neutron scattering, combined with electron microscopy, were employed to quantitatively characterize the precipitate structure, morphology, size distribution, and volume fraction. Although similar precipitate volume fractions and identical crystal structures were obtained after aging at 700 °C and 900 °C, distinct precipitation morphologies and sizes were developed, leading to markedly different mechanical properties. Aging at 700 °C produced a dual-scale structure consisting of fine spherical and coarser rod-like precipitates, contributing ∼500 MPa to the yield strength while causing a pronounced reduction in ductility. In contrast, aging at 900 °C resulted primarily in coarsened spherical precipitates, providing a lower strengthening increment of ∼254 MPa, while exhibiting higher ductility than the 700 °C-aged condition. These results clearly demonstrate that precipitate size and morphology exert a decisive influence on the strength–ductility trade-off, outweighing the effect of volume fraction alone, and thereby provide important guidance for microstructural design in precipitation-strengthened medium-entropy alloys.